1. Field of the Invention
The invention relates to a method for producing a digital sine wave signal with a given sampling rate, and a circuit configuration for carrying out the method.
A subscriber telephone line is supplied from a direct voltage source, which is connected in series with other signal voltage sources, for instance for speech signals in a phonic band, for wakeup signals, and for signals for indicating or displaying fees charged to the subscriber.
The connections of the lines are connected mutually parallel to both the speech circuit and the wakeup circuit of the subscriber telephone set, and are optionally connected, inside or outside the telephone set, to other devices, such as a display unit for progressively counting the charges as a telephone call is being made, which are constructed with the corresponding central station equipment. Switching equipment generates control signals that are normally called "charge or fee signals", and outputs them to the appropriate telephone line.
Such charge signals are understood to include charge pulses at a frequency of 12 kHz or 16 kHz, which are transmitted, with a duration of approximately 200 ms, during a call over a subscriber line in order to further increment the charge display by one unit. In order not to interfere with nearby frequency ranges, those charge pulses must be turned on and off softly. Transient response curves with a Gaussian function, for instance, are suitable for that purpose.
From German Published Non-Prosecuted Application DE 35 22 077 A1, corresponding to U.S. Pat. No. 4,720,852, and German Published Non-Prosecuted Application DE 35 16 007 A1, corresponding to U.S. Pat. No. 4,638,122, circuit configurations for signal generators for generating such charge signals by using analog circuitry are known.